A type of solar energy plant has emerged to compete with photovoltaic (PV) systems. PV systems can only generate power when the sun is shining. More recently, battery systems are being developed to allow PV systems to store excess energy for use when the plant is not generating electricity.
Concentrated Solar Power (CSP), or CSP technology heats a liquid circulating in the tubes of a heat exchanger called a collector, located on a tower using 2 axis controlled mirrors that have a focal length sufficient to concentrate the sun's rays on the heat exchanger. These mirrors have a slight curvature to enable the sun to be focused and concentrated onto the heat exchanger.
The liquid circulating within the tubes of the heat exchanger can be any liquid that is capable of absorbing heat. CSP plants have been built that utilize molten salt, or ethylene glycol as heat transfer fluids. The heated fluid, typically around 1200° F., passes through a heat exchanger used to generate superheated steam. This steam product is used to drive turbines connected to generators for the production of electricity.
The system is sized to allow it to collect significantly more heat during daylight hours than the power generation system can use. The excess heated fluid may be stored after the sun goes down and power may be generated from the heat stored in the fluid during the overnight hours. CSP plants are able to generate better returns on the owner's investment because the plants run 24 hours/day, even though the towers only operate when the mirrors can collect the energy from the sun. CSP plants can also be constructed next to traditional power generating plants such as nuclear, coal and natural gas, which all utilize the same steam turbine system to generate power and used to supplement steam production to the plant.
CSP plants are steam generation operations. Other applications that require high volumes of steam like salt water desalination by distillation, petroleum processing, mining operations and hydroponic vegetable growing are all candidates for this green technology.
Since the inception of CSP, it has become important to develop ways to keep the mirrors clean in order to keep the plants running efficiently. In order to maximize the energy production from a CSP plant, it would be desirable to provide a system to keep the mirrors as free of dirt and other debris, that prevent the sun's energy from reaching the tower. Mirrors are a piece of glass with a reflective coating on the back side. Soiling occurs on the top side from natural sources at a rate of ½-1% loss in reflectivity per day. The sunlight has to pass through the soils on the surface, then back through the soils on its reflective path to the collector. Thus a double effect on the hindrance. Tower collector efficiency has a direct 1:1 correlation on the loss of efficiency. A 10% loss in reflectivity is a 10% loss in energy collected by the system.
Specialized equipment has been developed to collect the sun's rays. These collectors are referred to in the industry as heliostats. Heliostats are structures that incorporate one or more curved sheets of glass, and support the glass surfaces so that they can be rotated about a first vertical axis, and about a second horizontal axis, such that they can be positioned in a manner to optimize the collection of solar energy.
One known heliostat design is manufactured by Brightsource Corp., and can be found at www.brightsource.com. The base of the heliostat is positioned in a hole in the ground and is surrounded by earth or may be encased in concrete. The base portion is stationary. Attached to the base portion is a moveable upper portion that rotates about a central vertical axis Y of the base, and also rotates about horizontal axis X by means of a motor driven linear actuator. The glass surfaces, due to having a required focal length based on their distance from the tower, are not flat but have a slight concave profile.
During operation, the glass surfaces make slight movements on both axis' every 1-5 minutes to maintain their direction of sunlight to the collector.
When the plant is not collecting solar energy, the glass surfaces are stored in a substantially horizontal position to minimize wind shear and loads to the glass and heliostat structure.
Previous attempts to keep the mirror surfaces clean include mechanical wet washing systems, electrostatic non-contact systems and films and coatings. The wet washing systems require water, and energy to pump the water and run the equipment. The runoff water has adverse effects to the environment, which is typically dry desert areas. The water fosters the growth of vegetation, which may need to be removed periodically. The vegetation bring insects, the insects bring birds and birds bring excrement droppings on mirrors which inhibit reflectivity. Water in arid climates is a valuable resource that shouldn't be wasted.
Coatings seek to prevent dirt and dust from sticking in the first place, but have relatively short life due to the constant high exposure of UV radiation from the sun. Recoating is expensive which makes it a cost prohibitive solution.
Electrostatic systems keep the surface clear of particulate debris but fail to prevent or remove film soiling buildup which occurs from moisture and pollutants in the air coming in contact with the cooler glass surface.
For these and other reasons, there is a need for the present invention.